Symmetries in Nuclei, Tokyo, 2008 Symmetries in Nuclei Symmetry and its mathematical description The role of symmetry in physics Symmetries of the nuclear shell model Symmetries of the interacting boson model

Symmetries in Nuclei, Tokyo, 2008 The nuclear many-body problem The nucleus is a system of A interacting nucleons described by the hamiltonian Separation in mean field + residual interaction:

Symmetries in Nuclei, Tokyo, 2008 The nuclear shell model Hamiltonian with one-body term (mean field) and two-body interactions: Entirely equivalent form of the same hamiltonian in second quantization: ,  : single-particle energies & interactions ijkl: single-particle quantum numbers

Symmetries in Nuclei, Tokyo, 2008 Boson and fermion statistics Fermions have half-integer spin and obey Fermi- Dirac statistics: Bosons have integer spin and obey Bose-Einstein statistics: Matter is carried by fermions. Interactions are carried by bosons. Composite matter particles can be fermions or bosons.

Symmetries in Nuclei, Tokyo, 2008 Boson-fermion systems Denote particle (boson or fermion) creation and annihilation operators by c + and c. Introduce (q=0 for bosons; q=1 for fermions): Boson/fermion statistics is summarized by Many-particle state:

Symmetries in Nuclei, Tokyo, 2008 Algebraic structure Introduce bilinear operators Rewrite many-body hamiltonian

Symmetries in Nuclei, Tokyo, 2008 Symmetries of the shell model Three bench-mark solutions: No residual interaction  IP shell model. Pairing (in jj coupling)  Racah’s SU(2). Quadrupole (in LS coupling)  Elliott’s SU(3). Symmetry triangle:

Symmetries in Nuclei, Tokyo, 2008 Racah’s SU(2) pairing model Assume pairing interaction in a single-j shell: Spectrum 210 Pb:

Symmetries in Nuclei, Tokyo, 2008 Pairing between identical nucleons Analytic solution of the pairing hamiltonian based on SU(2) symmetry. E.g., energies: Seniority  (number of nucleons not in pairs coupled to J=0) is a good quantum number. Correlated ground-state solution (cf. BCS). G. Racah, Phys. Rev. 63 (1943) 367 G. Racah, L. Farkas Memorial Volume (1952) p. 294 B.H. Flowers, Proc. Roy. Soc. (London) A 212 (1952) 248 A.K. Kerman, Ann. Phys. (NY) 12 (1961) 300

Symmetries in Nuclei, Tokyo, 2008 Nuclear superfluidity Ground states of pairing hamiltonian have the following correlated character: Even-even nucleus (  =0): Odd-mass nucleus (  =1): Nuclear superfluidity leads to Constant energy of first 2 + in even-even nuclei. Odd-even staggering in masses. Smooth variation of two-nucleon separation energies with nucleon number. Two-particle (2n or 2p) transfer enhancement.

Symmetries in Nuclei, Tokyo, 2008 Two-nucleon separation energies Two-nucleon separation energies S 2n : (a) Shell splitting dominates over interaction. (b) Interaction dominates over shell splitting. (c) S 2n in tin isotopes.

Symmetries in Nuclei, Tokyo, 2008 Wigner’s SU(4) symmetry Assume the nuclear hamiltonian is invariant under spin and isospin rotations: Since {S ,T,Y  } form an SU(4) algebra: H nucl has SU(4) symmetry. Total spin S, total orbital angular momentum L, total isospin T and SU(4) labels (, , ) are conserved quantum numbers. E.P. Wigner, Phys. Rev. 51 (1937) 106 F. Hund, Z. Phys. 105 (1937) 202

Symmetries in Nuclei, Tokyo, 2008 Physical origin of SU(4) symmetry SU(4) labels specify the separate spatial and spin- isospin symmetry of the wave function. Nuclear interaction is short-range attractive and hence favours maximal spatial symmetry.

Symmetries in Nuclei, Tokyo, 2008 Pairing with neutrons and protons For neutrons and protons two pairs and hence two pairing interactions are possible: 1 S 0 isovector or spin singlet (S=0,T=1): 3 S 1 isoscalar or spin triplet (S=1,T=0):

Symmetries in Nuclei, Tokyo, 2008 Neutron-proton pairing hamiltonian The nuclear hamiltonian has two pairing interactions SO(8) algebraic structure. Integrable and solvable for g 0 =0, g 1 =0 and g 0 =g 1. B.H. Flowers & S. Szpikowski, Proc. Phys. Soc. 84 (1964) 673

Symmetries in Nuclei, Tokyo, 2008 Quartetting in N=Z nuclei Pairing ground state of an N=Z nucleus:  Condensate of “  -like” objects. Observations: Isoscalar component in condensate survives only in N  Z nuclei, if anywhere at all. Spin-orbit term reduces isoscalar component.

Symmetries in Nuclei, Tokyo, 2008 (d,  ) and (p, 3 He) transfer

Symmetries in Nuclei, Tokyo, 2008 Wigner energy Extra binding energy of N=Z nuclei (cusp). Wigner energy B W is decomposed in two parts: W(A) and d(A) can be fixed empirically from binding energies. P. Möller & R. Nix, Nucl. Phys. A 536 (1992) 20 J.-Y. Zhang et al., Phys. Lett. B 227 (1989) 1 W. Satula et al., Phys. Lett. B 407 (1997) 103

Symmetries in Nuclei, Tokyo, 2008 Connection with SU(4) model Wigner’s explanation of the ‘kinks in the mass defect curve’ was based on SU(4) symmetry. Symmetry contribution to the nuclear binding energy is SU(4) symmetry is broken by spin-orbit term. Effects of SU(4) mixing must be included. D.D. Warner et al., Nature Physics 2 (2006) 311

Symmetries in Nuclei, Tokyo, 2008 Elliott’s SU(3) model of rotation Harmonic oscillator mean field (no spin-orbit) with residual interaction of quadrupole type: J.P. Elliott, Proc. Roy. Soc. A 245 (1958) 128; 562

Symmetries in Nuclei, Tokyo, 2008 Importance & limitations of SU(3) Historical importance: Bridge between the spherical shell model and the liquid- drop model through mixing of orbits. Spectrum generating algebra of Wigner’s SU(4) model. Limitations: LS (Russell-Saunders) coupling, not jj coupling (no spin- orbit splitting)  (beginning of) sd shell. Q is the algebraic quadrupole operator  no major-shell mixing.

Symmetries in Nuclei, Tokyo, 2008 Breaking of SU(4) symmetry SU(4) symmetry breaking as a consequence of Spin-orbit term in nuclear mean field. Coulomb interaction. Spin-dependence of the nuclear interaction. Evidence for SU(4) symmetry breaking from masses and from Gamow-Teller  decay.

Symmetries in Nuclei, Tokyo, 2008 SU(4) breaking from masses Double binding energy difference  V np  V np in sd-shell nuclei: P. Van Isacker et al., Phys. Rev. Lett. 74 (1995) 4607

Symmetries in Nuclei, Tokyo, 2008 SU(4) breaking from  decay Gamow-Teller decay into odd-odd or even-even N=Z nuclei. P. Halse & B.R. Barrett, Ann. Phys. (NY) 192 (1989) 204

Symmetries in Nuclei, Tokyo, 2008 Pseudo-spin symmetry Apply a helicity transformation to the spin-orbit + orbit-orbit nuclear mean field: Degeneracies occur for 4  = . K.T. Hecht & A. Adler, Nucl. Phys. A 137 (1969) 129 A. Arima et al., Phys. Lett. B 30 (1969) 517 R.D. Ratna et al., Nucl. Phys. A 202 (1973) 433 J.N. Ginocchio, Phys. Rev. Lett. 78 (1998) 436

Symmetries in Nuclei, Tokyo, 2008 Pseudo-SU(4) symmetry Assume the nuclear hamiltonian is invariant under pseudo-spin and isospin rotations: Consequences: Hamiltonian has pseudo-SU(4) symmetry. Total pseudo-spin, total pseudo-orbital angular momentum, total isospin and pseudo-SU(4) labels are conserved quantum numbers. D. Strottman, Nucl. Phys. A 188 (1971) 488

Symmetries in Nuclei, Tokyo, 2008 Test of pseudo-SU(4) symmetry Shell-model test of pseudo-SU(4). Realistic interaction in pf 5/2 g 9/2 space. Example: 58 Cu. P. Van Isacker et al., Phys. Rev. Lett. 82 (1999) 2060

Symmetries in Nuclei, Tokyo, 2008 Pseudo-SU(4) and  decay Pseudo-spin transformed Gamow-Teller operator is deformation dependent: Test:  decay of 18 Ne vs. 58 Zn. A. Jokinen et al., Eur. Phys. A. 3 (1998) 271

Symmetries in Nuclei, Tokyo, 2008 Three faces of the shell model